JP3356888B2 - Signal processing circuit of capacitance type sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal processing circuit for an electrostatic capacity sensor for detecting acceleration, pressure and the like.

[0002]

2. Description of the Related Art Conventionally, a piezoelectric sensor has been used as a sensor for detecting acceleration, pressure, and the like. Recently, however, a capacitance sensor utilizing a change in capacitance has been developed to provide high sensitivity and low power consumption. Attention has been paid to its advantages such as electric power (for example, Japanese Patent Laid-Open No. 19568/1992).

FIG. 7 shows the basic principle of this capacitance type sensor. FIG. 7 shows an application to an acceleration sensor. In the figure, (1) is a housing, and upper and lower two substrates are attached to the housing (1) at a predetermined distance from each other. The upper substrate (2) is a fixed substrate, and a circular fixed electrode (3) is attached to the center of the lower surface of the fixed substrate (2). On the other hand, the lower substrate (4) is a movable substrate made of a flexible material, and two fan-shaped movable electrodes (6) and (5) are provided at symmetrical positions on the upper surface of the movable substrate (4). Installed. Two fixed capacitors (8) and (7) are formed between the movable electrode (6) and the movable electrode (5) using the fixed electrode (3) as a common electrode. Further, a weight (9) is fixed to the center of the lower surface of the movable substrate (4).

In the acceleration sensor shown in FIG. 7, for example, when an acceleration is applied in the negative direction of the X-axis (left direction on the paper), the weight (9)
, An external force acts in the positive X-axis direction, and this external force causes a bending moment on the movable substrate (4), and the movable substrate (4) bends as shown in FIG. 7 (b). Due to this bending, the right movable electrode (5) approaches the fixed electrode (3), the left movable electrode (6) moves away from the fixed electrode (3), and the distance between the electrodes changes. 8) The capacitance value of (7) changes, and the acceleration is detected by detecting this capacitance change.

FIG. 3 shows a specific signal processing circuit of the acceleration sensor shown in FIG. In the figure, (10) and (20) are first and second delay circuits connected in parallel. Each delay circuit is composed of a resistor (11) (21) and a capacitor (7) (8). It is composed of an integrating circuit. Each of the first and second delay circuits (10) and (20) delays an input signal according to a time constant determined by a resistance value and a capacitance value with respect to a pulse input having the same phase. The resistance values of the resistance elements (11) and (21) of both delay circuits are set to the same value R, and the capacitors (7) and (8) of the acceleration sensor shown in FIG. 7 are connected as capacitors.

A phase comparator (30) shown in FIG. 3 compares the phases of the output signals of the first and second delay circuits (10) and (20). OR element "). Reference numeral (40) denotes a low-pass filter (referred to as LPF in the drawings) for smoothing the output of the phase comparator (30).

In the signal processing circuit shown in FIG. 3, a pulse signal Q0 generated from a pulse generator (not shown) is input to first and second delay circuits (10) and (20) through terminals. The input signal to each delay circuit is the delay circuit (10)
The delay is made according to the time constant determined by the resistance value R of each of the resistance elements (11) and (21) and the capacitance values Cx1 and Cx2 of the capacitors (7) and (8). Since the resistance values of the elements (11) and (21) are set to the same value R, the delay amount of the input signal changes according to changes in the capacitance values Cx1 and Cx2 of the capacitors (7) and (8) in the acceleration sensor. Will be.

Here, considering the state where no acceleration is applied to the acceleration sensor, the distance between the fixed electrode (3) and the movable electrodes (6) (5) constituting the left and right capacitors (8) and (7) is the same. Therefore, the capacitance values Cx1 and Cx2 of the capacitors (7) and (8) are the same, so that the delay circuits (10) and (20)
Are the same, and the input signals to the respective delay circuits are all delayed by an equal amount and output without a phase difference.
The phase comparator (30) composed of the EX-OR element compares the phase difference between the output signals from the delay circuits (10) and (20) and performs an exclusive OR operation. No output from the comparator (30) occurs. Therefore, no acceleration signal is generated from the low-pass filter (40), and it is determined that no acceleration exists.

Next, when acceleration is applied to the acceleration sensor in the negative direction of the X-axis, the movable substrate bends as shown in FIG.
The distance between one movable electrode (5) and the fixed electrode (3) is reduced, the capacitance value Cx1 of the capacitor (7) is increased, and the distance between the other movable electrode (6) and the fixed electrode (3) is increased. The capacitance value Cx2 of the capacitor (8) decreases. For this reason, a difference occurs between the delay amounts of the delay circuits (10) and (20). As shown in FIG. 4, the output Vc1 of the first delay circuit (10) is delayed by the delay amount D1 with respect to the input signal Q0. The output Vc2 of the second delay circuit (20) is delayed by a delay amount D2 with respect to the input signal Q0 (D1> D2). Note that the first and second delay circuits (1
Although the actual outputs Vc1 and Vc2 of (0) and (20) have integral waveforms, they are shown as pulse waveforms for convenience of explanation. Since a difference occurs in the delay amount in this way, a phase difference occurs between the outputs Vc1 and Vc1 of the two delay circuits (10) and (20).

The phase comparator (30) detects this phase difference,
A pulse signal Vx having a pulse width Tx corresponding to the phase difference is output, and this signal is smoothed by a low-pass filter (40).
It is output as an acceleration signal.

[0011] As the acceleration applied to the acceleration sensor increases,
The deflection of the movable substrate (4) increases, and the capacitor (7)
Since the capacitance difference of (8) also increases, both delay circuits (10)
The difference between the delay amounts in (20) also increases, the pulse width Tx of the output pulse Vx from the phase comparator increases, and the acceleration signal from the low-pass filter (40) also increases. Thus, the magnitude of the acceleration applied to the acceleration sensor can be detected.

However, in the signal processing circuit shown in FIG. 3, the output of the phase comparator (30) is zero in the initial state where no acceleration is present, and the two capacitors (7) when acceleration is applied. Since an output is generated in accordance with the difference in capacitance of (8), even if acceleration is applied in the positive direction or the negative direction of the X-axis, the phase is maintained as long as the capacitance difference between the two capacitors (7) and (8) is the same. The comparator (30) will produce the same output. For this reason, there was a drawback that the direction of the acceleration could not be detected.

Therefore, in order to detect the direction of acceleration, in an initial state where no acceleration exists,
It has been proposed to provide a difference between the time constants of the first and second delay circuits (10) and (20). Specifically, as shown in FIG. 5, the capacitor (8) in one delay circuit (20)
In parallel with the capacitor for generating a fixed reference phase difference (2
It has been proposed to connect 2) or to provide a difference between the resistance values of the resistance elements (11) and (21) although not shown. According to the processing circuit shown in FIG. 5, the waveform of each part in the initial state where no acceleration is present, as shown in FIG. 6, the delay amount D0 (corresponding to the capacitance value C0 of the reference phase difference generating capacitor (22)). Since the amount of delay of the input signal by the second delay circuit (20) is increased by the amount indicated by the waveform Vc2 in FIG. 6, a pulse output Tx0 is generated in the phase comparator (30).

In this state, when acceleration is applied in the negative direction of the X-axis in FIG. 7, the capacitance of the capacitor (7) increases, the capacitance of the capacitor (8) decreases, and the reference phase difference is generated. The difference between the capacitance values of the first and second delay circuits (10) and (20) including the capacitor (22) is reduced, and the pulse width of the output signal of the phase comparator (30) is changed to the initial pulse width T.
It is smaller than x0. Conversely, when acceleration is applied in the positive direction of the X-axis, the capacitance of the capacitor (7) decreases, the capacitance of the capacitor (8) increases, and the capacitance including the reference phase difference generating capacitor (22) is included. First and second delay circuits (10)
The difference between the capacitance values in (20) increases and the phase comparator (3
The pulse width of the output of (0) becomes larger than Tx0. As described above, since the pulse width of the output pulse of the phase comparator (30) decreases or increases depending on the direction of the acceleration, the acceleration signal from the low-pass filter (40) also increases or decreases from the reference value. The direction of the acceleration can be detected by the direction.

[0015]

However, in order to detect the direction of acceleration, a capacitor for generating a reference phase difference (202) is inserted into one of the delay circuits as shown in FIG.
In a signal processing circuit having a difference in resistance value, the configuration of both delay circuits except for the capacitors (7) and (8) by the acceleration sensor is different. For this reason, when the characteristics of the delay circuit change due to environmental conditions such as temperature, the change in the characteristics cannot be canceled out by the two delay circuits, and the difference in the characteristics is reflected in the amount of delay, and the low-pass filter (40) Has a disadvantage that the acceleration signal easily drifts, and the magnitude and direction of the acceleration cannot be accurately detected. For this reason, a compensation circuit is provided to offset the difference between the characteristics of the two delay circuits, and measures are taken to keep the environmental conditions constant.However, complicated circuits and high-precision parts are required. This leads to an increase in complexity of the processing circuit and an increase in cost.

The present invention has been made in view of the above-mentioned technical background, and not only can detect the magnitude and direction of acceleration, but also can detect them with a simple configuration and high accuracy. It is an object of the present invention to provide a signal processing circuit of a capacitance type sensor that can perform the processing.

[0017]

In order to achieve the above object, the present invention provides a reference phase difference detecting device for detecting the direction of acceleration by inserting a capacitor for generating a reference phase difference into a delay circuit or by changing a resistance value. Instead of forming the first and second
Is to form a phase difference in advance on the input signal itself to each delay circuit.

That is, according to the present invention, while the input signal is delayed, the capacitance values Cx1 and Cx2 of the two capacitors (7) and (8) whose input and output change in the opposite direction due to the detection external force are changed. The signal of a capacitance type sensor comprising two delay circuits (10) and (20) whose delay amount changes in accordance with the change of the phase and a phase comparator (30) for comparing the phase of the output signals of these delay circuits In the processing circuit, the two delay circuits (1
0) A signal processing circuit of a capacitance type sensor characterized in that a phase difference is provided in advance between input signals Q1 and Q2 to (20).

[0019]

According to the present invention, a reference phase difference for detecting the direction of acceleration is generated at the output of each of the delay circuits (10) and (20) based on the phase difference between the input signals Q1 and Q2. Therefore, it is not necessary to provide a capacitor for generating a reference phase difference in the delay circuits (10) and (20) or to provide a difference in resistance value, and the configuration of both delay circuits can be made the same except for the capacitor by the acceleration sensor. , Can offset the properties.

[0020]

Next, an embodiment of the present invention will be described with reference to FIGS. This embodiment shows a case where the present invention is applied to a signal processing circuit for the acceleration sensor shown in FIG. The same components as those of the conventional processing circuit shown in FIG. 3 are denoted by the same reference numerals.

In FIG. 1, (50) is a clock generating circuit for generating a clock pulse of a predetermined frequency, (60) is a counter, and the counter (60) receives the clock pulse from the clock generating circuit (50). The input signal is sent to the first and second delay circuits (10) and (20) in a state of receiving and receiving a preset reference phase difference φ.

The first and second delay circuits (10) and (20)
Is a resistor (11) (21) and a capacitor (7) respectively
(8) and an integrating circuit, and capacitors (7) and (8)
Each capacitor of the acceleration sensor shown in FIG. 6 is connected. These first and second delay circuits (10) and (20) are provided with respective resistance elements (11) (2) as in the prior art.
The resistance value of 1) and the capacitance value Cx1 of the capacitors (7) and (8),
The input signal is delayed by an amount corresponding to the time constant determined by Cx2. The resistance elements (11) and (21) have the same characteristic of the resistance value R.

(30) is a first and second delay circuit (10) (2)
0) is a phase comparator that compares the phases of the output signals and outputs a signal having a pulse width corresponding to the phase difference. In this embodiment, the phase comparator is configured by an EX-OR element. Therefore, the first and second delay circuits (10) are output from the phase comparator (30).
Of the output pulses in (20), one is at H level and the other is at L level.
A pulse which becomes H level when it is at level is output. That is, a pulse having a pulse width corresponding to the phase difference between the output pulses of the first and second delay circuits (10) and (20) is output.

Reference numeral (40) denotes a low-pass filter for smoothing the output of the phase comparator (30), and the output of the low-pass filter becomes an acceleration signal.

Next, the operation of the signal processing circuit shown in FIG. 1 will be described with reference to the signal waveform diagram of FIG.

The clock pulse generated from the clock generating circuit (50) is converted by the counter (60) into two input signals Q1 and Q2 having different phases, and the first and second delay circuits (10) and (20) ). In this example,
It is assumed that the input pulse Q1 to the first delay circuit (10) leads the input pulse Q2 to the second delay circuit (20) by a phase difference φ.

The input signals Q1 and Q2 to the first and second delay circuits (10) and (20) correspond to the resistance elements (11) and (21) of each delay circuit.
Are delayed by the delay amounts D1 and D2 corresponding to the time constants determined by the resistance value R and the capacitor capacitance values Cx1 and Cx2. Although the actual outputs of the delay circuits (10) and (20) are integrated waveforms, they are pulse outputs for convenience of explanation, and are represented by VC1 (output of the first delay circuit (10)), VC2 in FIG.
(Output of the second delay circuit (20)). When no acceleration is applied to the acceleration sensor, the two capacitors (7) and (8) of the acceleration sensor have the same capacitance, and the resistance values of the delay circuits (10) and (20) are set equal. , The time constants are the same. Accordingly, the delay amounts D1 and D2 are substantially the same, and the phase difference φx between VC1 and VC2 is φ which is the same as the phase difference of the input signal.

The outputs of the first and second delay circuits (10) and (20) are subjected to an exclusive OR operation by an EX-OR element as a phase comparator (30).
A pulse signal Vx0 that goes high when one is high and the other is low is output. The pulse width Tx0 of the pulse signal Vx0 corresponds to the phase difference φ.

Then, the output pulse V of the phase comparator (30)
x0 is smoothed by the low-pass filter (40), and serves as a reference signal when no acceleration is present.

Next, when an acceleration in the negative direction of the X-axis is applied to the acceleration sensor shown in FIG. 7, the weight (9) receives an external force in the positive direction of the X-axis, and the movable substrate (4) becomes as shown in FIG. Bends into
The capacitor (7) on the positive side of the X axis decreases the distance between the electrodes and increases the capacitance Cx1, and the capacitor (8) on the negative side of the X axis
In this case, the distance between the electrodes increases and the capacitance Cx2 decreases. As a result, the time constant of the first delay circuit (10) is large, and the time constant of the second delay circuit (20) is small. Accordingly, the delay amount D1 of the first delay circuit (10) with respect to the input pulse Q1 is large, and the delay amount D2 of the second delay circuit (20) with respect to the input pulse Q2 is small, and the output V of the first and second delay circuits is reduced.
The phase difference φx between c1 and Vc2 becomes smaller than φ. As a result, the pulse width Tx of the output pulse (indicated by Vx in FIG. 2) of the phase comparator (30) is reduced, so that the level of the acceleration signal from the low-pass filter (40) is also lower than the reference value. As the acceleration applied to the acceleration sensor increases, the first,
Phase difference φ between outputs Vc1 and Vc2 of second delay circuits (10) and (20)
Since x becomes smaller, the level of the acceleration signal from the low-pass filter (40) becomes smaller.

On the other hand, when acceleration in the positive direction of the X axis is applied to the acceleration sensor, the weight (9) receives an external force in the negative direction of the X axis, and the movable substrate (4) bends in the opposite direction to the above. In the capacitor (8) on the negative side, the distance between the electrodes is reduced and the capacitance Cx2 is increased, and in the capacitor (7) on the positive side on the X axis, the distance between the electrodes is expanded and the capacitance Cx1 is reduced. Thereby, contrary to the above, the outputs Vc1 and Vc2 of the first and second delay circuits (10) and (20)
Is larger than φ. As a result, the pulse width Tx 'of the output pulse (indicated by Vx' in FIG. 2) of the phase comparator (30) increases, and therefore the level of the acceleration signal from the low-pass filter (40) rises above the reference value. As the acceleration applied to the acceleration sensor increases, the phase difference φx between the outputs Vc1 and Vc2 of the first and second delay circuits (10) and (20) increases, so that the level of the acceleration signal from the low-pass filter (40) also increases. .

As described above, since the level of the acceleration signal from the low-pass filter (40) rises or falls from the reference level depending on the direction of the acceleration applied to the acceleration sensor, the direction of the acceleration can be detected. Of course, the magnitude of the acceleration can be detected from the level of the acceleration signal itself.

Further, in the processing circuit shown in FIG.
Since the resistance elements in the second delay circuits (10) and (20) have the same characteristics, even if the environmental conditions of the processing circuit, such as temperature, change, it is possible to cancel the characteristic change based on the resistance elements. it can. Therefore, the amount of delay of each input pulse by the first and second delay circuits depends only on the capacitances Cx1 and Cx2 of the capacitors (7) and (8) in the acceleration sensor, and changes depending only on the acceleration applied to the acceleration sensor. .
Therefore, there is no disadvantage that the acceleration signal from the low-pass filter (40) drifts due to environmental conditions such as temperature, so that it is not necessary to provide a drift compensating circuit or take measures for keeping the environmental conditions constant. It is possible to detect a great acceleration. Also, since the pulse generation circuit (50) and the counter (60) can be configured by digital circuits,
The circuit configuration of the entire processing circuit can also be simplified. In addition, since one pulse generating circuit (50) and a counter (60) are used to generate the input signals Q1 and Q2 having different phase differences,
Even if the environmental conditions change, the phase difference between the input signals does not change, and the change in the characteristics of the delay circuits (10) and (20) is canceled out. It becomes possible.

In the above embodiment, the phase comparator (3
Although the EX-OR element is used as 0), the present invention is not limited to this, and an output signal corresponding to the phase difference between the output signals from the first and second delay circuits (10) and (20) is generated. Anything is fine. Further, the case where the present invention is applied to an acceleration sensor for detecting acceleration is shown, but the present invention can also be applied to a sensor for detecting external force such as magnetism.

[0035]

According to the present invention, as described above, the input signal is delayed, and the amount of delay is changed in accordance with the change in each capacitance value of the two capacitors whose increase and decrease in capacitance value are oppositely changed by receiving a detected external force. In a signal processing circuit of an electrostatic capacitance type sensor comprising two delay circuits, the phase of which changes, and a phase comparator for comparing the phases of the output signals of the delay circuits, the input signal to the two delay circuits is Since the phase difference is provided, a reference phase difference for detecting the direction of the acceleration can be generated in the output of each delay circuit based on the phase difference of the input signal. Therefore, there is no need to provide a capacitor for generating a reference phase difference or provide a difference in resistance value in the delay circuit, and the configuration of both delay circuits can be the same except for the capacitor by the acceleration sensor. As a result, even if the environmental conditions of the processing circuit, for example, the temperature or the like change, the change in the characteristics of the two delay circuits can be offset, so that a compensating circuit for compensating for the difference in the characteristics is provided, It is possible to accurately detect the magnitude and direction of the acceleration with a simple configuration without requiring any countermeasure for performing the operation.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a signal processing circuit of a capacitance type sensor according to one embodiment of the present invention.

FIG. 2 is an operation waveform diagram of the signal processing circuit of FIG. 1;

FIG. 3 is a block diagram showing a signal processing circuit of a conventional capacitive sensor.

FIG. 4 is an operation waveform diagram of the signal processing circuit of FIG. 3;

FIG. 5 is a block diagram showing a signal processing circuit of a conventional capacitive sensor.

FIG. 6 is an operation waveform diagram of the signal processing circuit of FIG. 5;

7A and 7B show an example of a capacitance type sensor, and FIG.
Is a cross-sectional view in a state where no acceleration is present, (b) is a cross-sectional view in a state where acceleration is present, and (c) is a VIc− in (a).
It is a VIc line sectional view.

[Explanation of symbols]

 10, 20 delay circuit 7, 8 capacitor 30 phase comparator

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01P 15/125 G01L 9/12 H01L 29/84

Claims (3)

(57) [Claims] An input signal is delayed, and a change in capacitances Cx1 and Cx2 of two capacitors (7) and (8) in which an increase and a decrease in capacitances Cx1 and Cx2 change reciprocally in response to a detected external force. Delay circuits with variable delay amount (10)
(20) and a phase comparator (30) for comparing the phases of the output signals of these delay circuits. In the signal processing circuit of the capacitance type sensor, the two delay circuits (10) and (20) Input signals Q1, Q
2. A signal processing circuit for a capacitive sensor, wherein a phase difference is provided in advance in the signal processing circuit. 2. A phase difference between input signals Q1 and Q2 is determined by a counter.
The capacitance type according to claim 1, which is set by (60).
Sensor signal processing circuit. 3. The phase comparator (30) is an exclusive OR element.
The capacitance type sensor according to claim 1,
Signal processing circuit.